Sargassum polycystum had been widely used in food products as a gelling agent and thickening agent. However, the antibacterial component of this alga was rarely used optimally. The purposes of this study were 1) to determine the antibacterial activity from Sargassum polycystum using different combination of solvent; 2) to determine the optimum pH and heating temperature of the antimicrobial activity; 3) to determine antimicrobial compound in Sargassum polycytum using GC-MS; and 4) to determine the damage of the bacterial cell using SEM.. Solvent that used in extraction are ethanol and ethyl acetate with ratios 100:0, 80:20, 60:40, 40:60, 20:80, and 0:100. The microbes that used in this test were S. ausreus, B. cereus and E. coli. In this study, the antibacterial activity was determined by the diffusion wells, the MIC & MBC and total phenolic. The highest zona of zone inhibition, the lowest MIC & MBC, and the lowest total phenolic value were given by using ethanol and ethyl acetate solvent (ratio 0:100). Both the pH and heating temperature affected the antibacterial activity. The combination of pH used in this study was pH 3, 4, 5, 6, 7, and 8 with temperatures of 40oC, 50oC, 60oC, 70oC and 85oC. The highest antibacterial activity within the extraction Sargassum polycystum occurred with pH 3 and a temperature of 30oC for E. coli and pH 5 and a temperature of 26oC for S. aureus. The component indicated by GS-MS was fatty acids, which acted as antimicrobials in extract Sargassum polycystum. Cell damage indicated by SEM consisted of cell wall damage, cell leakages and deformation of cells.